Work apparatus such as chain saws, cutoff machines, brushcutters or the like include a drive train which, in turn, includes an internal combustion engine, a disconnect clutch and a tool which is driven by the engine in the engaged state of the disconnect clutch. This system includes a series of nonlinear elasticities such as the load-dependent changeable elasticity of a saw chain. The elasticity in combination with the mass of the drive train defines a system capable of vibration which is excited toward vibration because of torque fluctuations of the drive motor caused by nonuniformly worn cutting teeth or the like. Especially at operating rpm, disturbing subharmonic vibrations become noticeable at the handle of such a work apparatus. These vibrations cause the apparatus to vibrate and are transmitted to the handles. These handle vibrations reduce the comfort with which an operator works with the apparatus and can lead to a premature tiring of the operator. Furthermore, pulse-like force peaks occur in the drive train because of nonuniformities in the workpiece and/or because of errors in the use of the apparatus. These force peaks lead to a high loading of the components such as bearings and clutch entraining elements.
It is an object of the invention to provide a work apparatus wherein the working comfort of the apparatus is increased.
The portable handheld work apparatus of the invention includes a motor-driven chain saw, cutoff machine and a brushcutter. The work apparatus includes: a housing; an internal combustion engine mounted in the housing; a work tool; a centrifugal clutch interconnecting the engine and the work tool to permit the engine to drive the work tool; the centrifugal clutch including a carrier driven by the engine; a plurality of centrifugal weights mounted on the carrier; an output part having a contact wall adjacent the centrifugal weights; the plurality of centrifugal weights being mounted so as to be displaceable between a first position wherein the centrifugal weights are disengaged from the contact wall and a second position wherein the centrifugal weights are in torque-transmitting contact engagement with the contact wall; the centrifugal clutch further including means for resiliently biasing the centrifugal weights into the first position; an intermediate member connected to the work tool so as to cause the work tool to rotate therewith; and, a rotationally-elastic damping element disposed between the intermediate member and the output part so as to operatively connect the intermediate member and the output part to each other.
The vibration system of the drive train is adapted in a simple manner and so as to save space and weight such that the subharmonic vibration excitation is reduced at operating rpm by the assembly of the centrifugal clutch with an output part, an intermediate element and a rotation-elastic damping element arranged between the output part and the intermediate element. In this way, the level of vibration at the handle is significantly reduced and the operating comfort of the work apparatus is thereby increased. A work apparatus configured in accordance with the invention is also more insensitive to vibration excitation because of nonuniformly worn rotatable work tools such as knives in the case of a brushcutter or the like. Because of the increased tolerance of the system against vibration excitation, harder anti-vibration elements are possible between the engine and the handle which makes guidance stiffer. In this way, the precision of the guidance of the work tool by the operator is improved. Furthermore, the dynamic load on the bearings and on the clutch is reduced and therefore a premature wear is prevented.
A rigid coupling of the drive part of the clutch to the engine is provided by the arrangement of the damping element on the output end of the centrifugal clutch. In this way, the spring element is ineffective in the disengaged state of the clutch and the rigidly connected mass of the drive part can, as a balancing mass, contribute to a quiet running of the engine in idle.
In a preferred embodiment of the invention, the output part in the centrifugal clutch is configured as a cylinder ring which, on its periphery, has one or several damping elements and is thereby connected rotationally elastically to the clutch drum. The cylinder ring assumes the function of the peripheral wall of a conventional clutch drum and transmits the torque by means of the peripherally arranged damping elements to the clutch drum enclosing the cylinder ring and the damping elements. The clutch drum is enlarged correspondingly with respect to diameter. The clutch drum defines the intermediate member for the rotationally fixed connection, for example, of the chain sprocket of a chain saw or of the tension flange for the drive shaft of a brushcutter. With this arrangement, no additional structural space in the axial direction is required for the damping elements. Furthermore, the damping elements can be easily and simply configured because they are arranged close to the periphery with a large radius and therefore are loaded relatively little.
To avoid an additional journalling of the cylinder ring, the cylinder ring preferably includes radial ribs which project outwardly and are distributed uniformly in the peripheral direction. These ribs are supported against the inner side of the peripheral wall of the clutch drum. A uniform centering of the cylinder ring without further journalling is given especially in combination with radial ribs of the peripheral wall of the clutch drum which extend inwardly and which, in turn, are supported against the cylinder ring. The ribs serve also to support the damping elements whereby additional devices to connect the same are unnecessary.
The damping elements are preferably configured as flat springs and especially as meander-shaped wire springs which are braced with their ends against corresponding ones of the ribs. In this way, only a small structural height is required in the radial direction. The spring and damping characteristics can be adjusted with simple means within wide limits with the selection of the wire thickness, number and form of the meander-shaped angles as well as sheet metal strips or plastic strips placed optionally in the angles of the spring. This adjustment of the spring and damping characteristics is achieved without it being necessary to change the main dimensions of the coupling cylinder and the clutch drum. An adaptation of the peripheral end applied force on the wire springs and therefore a friction damping can be adjusted in a simple manner via the selection of different bending radii in the peripheral direction.
The damping elements are built in purposefully with a pretensioning in such a manner that they, in the unloaded state of the disconnect clutch, press the ribs of the cylinder ring against the ribs of the clutch drum in opposition to the direction of rotation. In this way, a rigid and therefore precise connection of the tool to the engine is provided during operation at low torque. The pretensioning force of the spring element is overcome when torque peaks caused by vibration are present and thereby the torque behavior is smoothed.
In an advantageous embodiment, the intermediate member is arranged in the form of a disc at the end face toward the clutch drum. The clutch drum is here configured as the output part. The damping elements are provided in the axial direction between the end wall of the clutch drum and the disc. In this way, no additional radial structural space is needed for the wire elements so that, if necessary, present clutches and apparatus housings require only minor constructive retrofitting and, if required, even a retrofit of already manufactured work apparatus is possible.
Lugs are arranged on a circle on the end wall of the clutch drum and, opposite thereto, on the output part. These lugs are connected by means of the damping elements. These damping elements too are preferably configured as flat springs and especially as meander-shaped springs. In this way, a simple assembly without tools is possible while at the same time requiring little space.
The two groups of lugs can be advantageously connected by means of wire springs because of the arrangement of a number of lugs on a circular line close to the periphery of the clutch and a corresponding number of lugs arranged along a circular line of lesser radius. These two groups can be connected in that, on the one hand, a high carrying capacity is given by the large radius and, on the other hand, a high number of wire springs can be accommodated because of the radial offset. Twice the number of bolts is provided on the outer circle compared to the number of bolts on the inner circle in order to reduce the number of parts. Two bolts of the outer circle are connected via a double spring clamp to a corresponding one of the bolts of the inner circle. In this way, a high carrying capacity can be obtained while simultaneously having a reduced number of parts and therefore a reduced complexity with respect to manufacture and assembly. The damping elements are preferably configured as steel springs permanently fixed with respect to vibration whereby a high carrying capacity is combined with reduced maintenance costs.